Negative feed-back transmission system



H. P. THQMAS .NEGATIVE FEEDBACK TRANSMISSION SYSTEM F1166. April 50,1947 Uhr;

. high modulating frequencies, is

Patented May 3, 1949 NEGATIVE FEED-BACK TRANSMISSION SYSTEM `Hem-y P.Thomas, Fayetteville, N. Y., assignor to General Electric Company,

New York a. corporation of Application April 30, 1947, Serial No.744,845

(Cl. S32-.18)

, 9 Claims.

1 My invention relates to a. transmission system employing negative ordegenerative feedback, and it has particular utility in a system forimproving the stability and fidelity of an angularlymodulatedtransmitter of the phase-or frequency modulation type.

It is an object of my invention to provide an improved degenerativefeedback system for increasing the stability and linearity-of a, signalchannel including both an amplifier which is supplied with modulationfrequency signals and a modulator which develops a high `frequencymodulated output wave.

Another object of 'my invention is to provide a selective feedbacknetwork for a high frequency modulation system which is particularlyeffective to stabilize the operation at lower modulation.

frequencies, thereby realizing the reduction in distortion and hum whichis characteristic of a negative feedback system. without adverselyaffecting the over-all gain-frequency characteristic of the system.

-Still another object of my invention is to provide an improvedselective negative feedback system which is particularly adapted to therequirements of a phase or frequency modulation network.

A still further object of my invention is to provide an improvedselective degenerative feedback network for a frequency modulationsystem in which undesirable regeneration, which might otherwise occurdue to excessive phase shifts at prevented Without adversely affectingover-all gain-frequency characteristics of the system. k

The features of my invention which I believe to be novel are set forthwith particularity in the appended claims. My invention itself, however,together with fur-ther objects and advantages thereof, may best beunderstood by reference to the following description taken in connectionwith the accompanying drawings, in which Fig. 1 is a circuit diagram,partly in simplified block form, illustrating one application of myinvention to a carrier wave transmitter of the angularly modulated type;and Fig. 2 is a generalized schematic diagram which will be referred toin analyzing certain fundamental principles underlying my invention.

In one embodiment of the present invention there is provided a radiotransmission system which may, for example, be of thefrequencymodulation type, employing negative feedback in the system. Afeedback network, for a translating system receiving a main vappliedsiga tendency for .applied to the input nal from a source, is a. networkadapted to apply to the input terminal of the system, along with .themain applied signal, a signal derived from the output terminal of thesystem. If the signal thus `fed back is opposed to the main signal, thatis, if one is increasing when the other is decreasing, then the feed:back network is acting as a` negative or degenerative feedback network.Asis well-known to the art, one of the various advantages of negativefeedback is that the linearity of the system is improved. That is, thereis more of a tendency for the output signal to be a faithfulreproduction of the main applied signal, rather than a distortedreproduction thereof.

In the absence of certain additional features to be mentioned,considerable diiculty would be encountered from the fact that undercertain conditions, more high frequencies, there is a tendency for thefeedback signal to said, rather than oppose the main applied signal, aswill lbe understood by those skilled in the art from considerationsofphase shift within the system. When the feedback signal aids the mainapplied signal, there is a condition of positive feedback orregeneration, Aand self-oscillation of the system, which isundersirableln this system.

One embodiment of the present invention may be viewed as havingsignal-translating means, a first network through which the main signalis terminal of the signal-translating means, and a second networkthrough which a signal is fed back from the output terminal to theinputterminal. The feedback network of the present invention is adapted t0feed back a smaller signal at high frequencies than at low frequencies,which is advantageous in avoiding self-oscillation at those highfrequencies where the feedback would become aiding or regenerative. If asmaller signal is fed back at one moment than at another moment,however, there would be a tendency for the total am plication of thesystem to vary, which would be undesirable. As a special feature of oneembodiment of the presentinvention, there is provided another networkwhich is interposed between the source of the main signals and the inputterminal of the vsignal translating means, and this lastmentionednetwork is adapted to control the amplitude of the main signal appliedto the input terminal of the signal-translating means so that arelatively large frequencies when the negative feedback signal is large,and a, smaller signal is applied at higher particularly at certainrelatively main signal is applied at lowr frequencies when the negativefeedback signal is small. Since the main applied signal and the feedbacksignal are opposed to one another, the net applied signal and hence theoutput signal are determined by their difference. lation of the twonetworks may be adjusted so that the aplification of the entire systemis constant under various conditions of operation. Hence negativefeedback is utilized with its attendant advantages, whileself-oscillation and variations in the amplification are prevented, asdesired. v

In the radio transmitter illustrated in Fig. 1, audio modulation signalsare impressed upon the input terminals I0 of audio frequency amplifierII from any suitable signal source. not shown. This source may, forexample, comprise a studio microphone and suitable pre-amplifiers and-audio pre-emphasis networks commonly employed in frequency modulationbroadcasting practice. The amplified audio signals appearing across theanode load resistor 22 through blocking capacitor I3 upon a frequencyselective network serially including resistors I4, potentiometer I5 andcoupling capacitor I6, the function and operation of which will bedescribed in detail presently. Potentials appearing across the lowerportion of potentiometer I5 and across capacitor IIB, between anadjustable ta-p I1 and-ground, are impressed upon the control grid of asecond audioamplifier I2. A direct current grid-return is provided inshunt yto the capacitor I6 by the relatively high resistance I8 andsuitable operating v bias for'the amplifier I2 is provided inconventiona1 manner by the cathode bias resistor I9 shunted by the usualaudio bypass capacitor 20.

The remaining proper may be entirely conventional and are thereforerepresented only in simplified block form. The yamplied audio signals atthe output of amplifier I2 may be further amplified in an audio poweramplifier before they are iml pressed upon a frequency modulator 26. The

modulator 26 may be any one of a number well knownto the art. 'Forexample, it may comprise a modulation system of the type shown inapplication Serial No. 632,977, Francis M. Bailey, filed December 5,1945, now Patent No. 2,461,250, issued\February 8, 1949, and assigned tothe same assignee as the present invention. In this type of system, aspecial tube and circuits is used for modulator 26 and the mean carrierfrequency is controlled'by means of a crystal oscillator 21.

The singularly-modulated output wave from modulator 26 is multiplied toa suitable carrier frequency in conventional manner by a plurality offrequency multipliers, indicated by the blocks 28 and 29. f Themodulated carrierwave is then `amplified in the radio frequencypowerampliners 30 before being 'supplied to the radiating antenna 3l.

In accordance with my invention, a degenerative or negative feedbackpath is provided between a point in the high frequency channel'following the modulator 26 and the input of the audio amplifier I2. Theinput to 4the feedback network may be supplied from any suitable pointat which the frequency is wave may be readily amplified and-demodulated,and so connected that the polarity is such that the feedback to amplierI2 is degenerative. As shown in Fig. 1, it is supplied from a point 32between the frequency multipliers. 28 and 29, each of which may have oneor more stages.

are impressed circuits of the transmitter y fact, I have found inpractice current lament supply for modulator 26, presuch that thecarrier i The signals may be first amplified in a conventional highfrequency amplifier 35, and are then supplied to a well known form offrequency discriminator network 36. As illustrated, this discriminatoris of the type shown in Patent 2,121,103-S. W. Seeley, granted June 21.1938, although many other suitable types are known to the art.

The demodulation` components are ksupplied through blocking capacitor 31to a second frequency selective network which comprises resistor 38 andcapacitor I6. Capacitor I6 therefore acts as a common coupling impedancefor both the audio input signals and the feedback signals supplied tothe grid of amplifier l2.

Since a considerable amount of phase shift at the higher moduatingfrequencies is inevitably introduced by the high frequency circuits ofthe modulator 26 and following circuits, the feedback may becomeregenerative at these frequencies'even though the feedback network issupplied from a proper point to provide degenerative phase, at the gridof amplifier I2, at the lower modulating frequencies. This of coursecannot be toleratedbecause it will produce circuit instability andparasitics, defeating the purpose of the feedback network. Consequently,the frequency selective nework 38--I6 is employed to provide again-frequency characteristic which varies substantially inversely overthe audio frequency of the type shown in the aforesaid Baileyapplication, or an Armstrong phase modulator, the audio frequencydistortion is a function of the angular excursion caused bythe appliedmodulating signal. ulation of the entire frequency modulation system,the angular inversely proportional to the modulating frequency. Hence,the most severe distortion occurs at the lowest modulating this reason,the use of negative feedback at only the lower frequencies is stillfully effective in rewhich are commonly encounthe use of alternathumfrequencies, tered in transmitters due to ing current lament supplies,is also generally most severe at the lower harmonics of the power supplyfrequency, such -at cycles and 360 cycles per second', for example.Therefore the negative feedback at lower audio frequencies is alsoeffective in suppressing these effects. In that the direct viouslyrequired, canbe eliminated and alternating current supplies used'throughout the transmitter.

As a practical matter, I have found that It ,is satisfactory to designthe frequency selective network 38-I6 to attenuate substantially thehigher modulating to 1000 cycles per second but to pass frequenciesbelow this value without' substantial attenuation. Expressed anotherway, the time constant of this network, that is, the product of itsresistanceby its capacity, may correspond to the period of a mediumaudio frequency of the order of about 400 lto 1000 cycles per second.However, when the feedback is reduced or cut off at the highermodulating frequencies, the relative gain and degree of modulationincreases. In accordance with my invention, this is corrected bytheinverse frequency characteristic network in the audio input to the gridof ampliner I2, comprising vthe frequency selective network,

range. In a modulation system For a fixed percentage modexcursion of themodulator is frequency. For

frequencies above about 400- M-IS-i. If the time constant of this net- Awork is made equal to that of the network 38-IB, that is, if theresistors I4 and i5 in series have the same resistance as resistor 38,then the tap I'l may be adjusted so that the over-al1 gain 5 i plingimpedance Z. Resistance R1 corresponds substantially to resistor i4 inFig. 1 plus that portion of potentiometer I5 above tap I'I (indicated bypoint lla in'Fig. 2), assuming that the source Il is a substantiallyconstant voltage feed. Resistance R2 corresponds to that portion ofpoteniometer I5 below tap I1, and impedance Z corresponds to capacitorI6. The entirersignal translating network between the input to amplifieri2 and the feedback 32 is represented by the block 4| which is indicatedas having an equivalent over-al1 gain y. at modulation frequencies. Thefeedback network, corresponding to the amplifier 35 and discriminator 36of Fig. l, is represented in Fig. 2 by the block 42 which is indicatedas having an equivalent over-all backward gain ,c at modulationfrequencies. The second frequency selective network is represented bythe resistance R and the common coupling impedance Z in which resistanceR corresponds to resistor 38 in Fig. 1. The relatively. high resistanceI8 in Fig. 1, which provides a direct current return path between thegrid and cathodel of amplifier I2, has substantially no effect upon theoperation of the feedback system.

Now if the resistances R and R1 are both made high relative to theresistance of R2, the potential of point Ila can be considered to beessentially the same as that of the Upper terminal 2| of the commoncoupling impedance simplifying the network analysis.

With the above definitions and assumptions in mind, the gain of thesystem, with feedback, between points Ila and 32.l is given by theexpression:

G1 pl where a is the gain of the system in the forward direction withoutfeedback, as previously described, and 1 is thegain of the totalfeedback circuit. If the system had a gain independent of frequencybefore the addition of feedback, ,u in

this expression will be a constant; but with selective feedback thetotal gain G1 will vary with fregain is then a constant 42) times thisexpression:

l R J-X The gain G1 of the system between points Ila and 32 in thepresence of feedback is then:

The total feedback (the gain of network The gain G2 through the networkR1, R2, Z is given by the expression:

RZ-XJL.

If R14-R2 is made equal to R so that the two frequency selectivenetworks have the same time constant, then:

G2: R JX v where:

R2 fC-a+ R, (8) Then the total system gain from signal source 40 to theoutput point 32 is:

K R -jXe G=G1G2=l%- (9) 1 R IYc KR c G* R-J'X. 1+ (1) I: R 1/ K (J'XJ GK[ R- 1 cXi (11) The gain will be independent of frequency if:

' 1 K m73 (l2) and will have a value:

quency. The network R1, Rz, Z is designed to provide a gain-frequencycharacteristic which is the inverse of that ofthe system with feedbackapplied, as will appear later. If the gain of this network isrepresented by Ga, then the complete gain of the system from the signalsource 40, to the output is represented by:

In other words, the gain-frequency characteristics of the system will berestored to the value without feedback if: f'

It is possible to use a considerablenumber of variations in thesenetworks and still maintain the desired inverse characteristics. Forinstance, it can be seen directly from Equation 11 that the simplecapacitor used for the. common -coupling impedance may be replaced by ageneral impedance Z and if the relations of Equations 12 and 14 arestill maintained, the gain will still be independent of frequency. Forinstance, the impedance Z might be a series circuit of inductance andcapacity which could be selected to give essentially infiniteattenuation in the feedback circuit at any particular desired frequencymerely by adjusting the series resonance of L and C so as to occur atthat frequency. This may be advantageous in reducing the gain of thefeedback circuits to a very low value at the frequency at which thephase shift through the system becomes degrees, thus preventinginstabilityvof the sytsem.

Merely by way of illustration, not in any sense by way of limitation, Ihave found the following values of circuit constants to be satisfactoryin Audio band width=5015,000 cycles per sec.

. Resistor 14=200,000 ohms Resistor 15==100,000 ohms Resistor 38=300,000ohms Capacitor 16:.0082 m-fd.

Resistor 18--1 megohm While I have shown a particular embodiment of myinvention, it will of course be understood that I do not wish to belimited thereto since various modifications may b e made, and Icontemplate by the appended claims to cover any such modcations as fallwithin the true spirit l and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is;

1. In a transmitter of angularly-modulated Ihigh frequency waves, asource ci modulation signals extending over a relatively. low-frequencyband, a, signal translation channel having an input connected to saidsource and'having an output, said channel including means for generatingsaid high frequency waves and 'modulating them in accordance with saidsignals, means for demodulating said angularly modulated waves at saidoutput to develop demodulated signals, a frequency-selective' networkhaving an inverse gain-frequency characteristic over said band, meansfor feeding back said demodulated signals through said network to saidinput in degenerative phase, whereby the relativedegree of modulation atsaid output is increased at higher modulation frequencies, and a secondfrequency-selective network in the connection between said source andsaid input including a coupling impedance across said input common toboth said networks, said second networkhaving a correlatedgain-frequency characteristic which substantially restores the relativedegree of modulation at said output to the value without feedback.

2. A modulation system comprising, in combination, a high frequencysignal modulator having a predetermined gain-frequency characteris ticand having an input and an output, a first frequency-selective network,means for impressing potentials within a band of modulation frequencieson said network, an impedance in said network coupled to said input, anegative feedback network comprising means for deriving potentials ,atmodulation frequencies from said output and a second frequency selectivenetwork, said second network also including said input couplingimpedance, each of saidnetworks having maximum gain at a relatively lowfrequency in said band and decreasing gain at higher modulationfrequencies, and means to adjust the relative amplitudes of saidpotentials impressed on said input from said frequency-selectivenetworks, whereby the effect of said feedback network upon saidgain-frequency characteristic at said output may be substantiallycompensated.

3. In a wave transmission system, a signal translating network having aninput and an output and a forward gain a over a predetermined operatingfrequency range, a frequency-selective network serially comprising arelatively high resistance R1, a relatively low resistance Rz and acoupling impedance, said input being connected across said resistance R3and said impedance in series, means for impressing lsignals within saidfrequency range across said network. a negative feedback network havinginput and output'terminals and having backward gain over said range, asecond frequency-selective network serially comprising a relatively highresistance R and said impedance, means coupling said output to saidinput terminals,

and means for impressing signals from said output terminals across said^second network, the

values of said resistances substantially satisfying the equations R=R1+Rz and R1+f'R'='1+' fi whereby the frequency response characteristic ofsaid system is .substantially unaffected by said 15 feedback over saidrange.-

V4. In a wave translating system, a signal translating path having again a between input and output terminals, a signal frequency sourcehaving an output, a first frequency-selective network composed of arelatively high resistance R1. a krelatively low resistance Ra and acoupling reactance in series, said input terminals-being coupled'acrosssaid resistance Rz and said reactance in series, means forimpressing signals from said source across said first network, and anegative feedback patnhaving a gain between its input and output,.saidinput being coupled to said output terminals and saidr output beingcoupled across a second frequency-selective network composed of arelatively high resistance R in series with said coupling reactance, inwhich both said networks have substantially the same time constant andin which-the relationship 1a. l. RVi-R, l-p

is substantially satisfi 5. In a modulation and comprising mean s todemodulate said waves and to develop demodulated signals lyingl withinsaid band, a second frequency selective network, means to supply saiddemodulated signals back to said input through said second network in degenerative phase, each of said frequency-selective networks havingsubstantially the same time,

constant and having an inverse gain-frequency characteristic over saidband, whereby the relative degree of modulation at said output tends tobe increased at higher frequencies within said band duetto saidfeedback, and means to adjust the relative amplitudes of said signalssupplied to said input from said first and second networks,

whereby the effect of said feedback network upon the relative degree o fmodulation may besubstantially compensated.

6. In a modulation system, the combination of a source of audiofrequency signals, .Ja signal translating channel having an input andanoutput, said channel comprising an audio amplier supplied from saidinput and a modulator for producing high frequency waves modulated bysaid signals at said output, a first frequency sel lective networkcomprising a potentiometer and a coupling impedance serially connectedacross said source, said network having a gain which varies inverselywith frequency over the audio system, the combination of range, meansfor impressing audio signals appearing across said impedance and anadjustable portion of said potentiometer upon said input, a negativefeedback network supplied from said output, said feedback networkincluding means for demodulating said waves and deriving therefrom theaudio modulation components, a second frequency-selectivenetworkserially comprising a resistor and said coupling impedance, and meansfor impressing said modulation components across said second network,said second network having substantially the same time constant as saidrst network. Y

7. In a wave transmission system, a signal translating network having aninput and an output, a frequency selective network serially comprising aresistor and a capacitor, means for supplying audio frequency signalsacross said network, means for impressing upon said input audio signalsappearing across said capacitor and a tapped point on said resistor, anegative feedback network comprising means for deriving signals withinthe audio frequency range from said output, a second frequency-selectivenetwork, and means for impressing said derived signals across saidsecond network, said second network serially comprising a secondresistor and said capacitor, both said frequency-selective networkshaving substantially the same time constant.

8. In a wave transmission system, a signal translating network having aninput and an output, a frequency selective network serially comprising aresistor and a capacitor, means for supplying audio frequency signalsacross said network, means for impressing upon said input audio signalsappearing across said capacitor and an adjustable tapped point on saidresistor, a negative feedback network comprising means for derivingsignals within the audio frequency range from said output, a secondfrequency-selective network, means for impressing said derived signalsacross said second network, said second network serially comprising asecond resistor and said capacitor, both said frequency-selectivenetworks having substantially the same time constant corresponding tothe period of a mediumfrequency audio wave, and means for adjusting saidtapped point to adjust the frequency transmission characteristics ofsaidsystem.

9. In a carrier wave transmission system, the combination of a signalchannel having input and output terminals, said channel comprising anAaudio ampliier supplied from said input terminals and a modulator forproducing high frequency waves modulated by said signals at said outputterminals, a source .of audio frequency signals, a firstfrequency-selective network serially consisting of a potentiometer and acoupling capacitor, means for impressing signals from said source acrosssaid rst network, means for impressing audio signals appearing acrosssaid capacitor and a point on said resistor upon said input terminals, asecond frequency-selective network serially consisting of a secondresistor and said coupling capacitor, a negative feedback networkincluding means for deriving signals within the audio frequency rangefrom said output terminals and impressing them across said' secondfrequency-selective network, both said frequencyselective networkshaving substantially the same time constant corresponding to the periodof a medium audio frequency, whereby the relative degree of modulationat said output terminals tends to be reduced at lower audio frequenciesdue to said feedback, and means for adjusting said point on said firstresistor to compensate for said tendency. L

HENRY P. THOMAS.

REFERENCES CITED The following references are of record inthe le of thispatent:

UNITED STATES PATENTS

